1. Field of the Invention
The present invention is related to a circuit for controlling load switch, and more particularly related to a circuit for controlling load switch by using three comparators and two switches to adjust the average current.
2. Description of the Prior Art
FIG. 1 is a circuit diagram showing a conventional integrated light emitted diode (LED) circuit. As shown, the conventional integrated LED circuit PA1 includes a load circuit PA11 and a driver circuit PA12. The load circuit PA11 is a LED circuit, and further includes a voltage source PA111, a full bridge rectifier circuit PA112, a resistor PA113, a diode PA114, a capacitor PA115, at least a LED PA116, an inductor PA117, a first switch PA118, a resistor PA119, a capacitor PA120, and a capacitor PA121.
The full bridge rectifier PA112 is coupled to the voltage source PA111 and coupled to the resistor PA113, the diode PA114, the capacitor PA115, and the LED PA116. The inductor PA117 has one end coupled to the diode PA114 and the drain of the first switch PA118 and another end coupled to the capacitor PA115 and the LED PA116. The resistor PA119 is coupled to the source of the first switch PA118 and also to CS pin of the driver circuit PA12. The capacitor PA120 is coupled to the resistor PA113 and also to VCC pin of the driver circuit PA12. The capacitor PA121 is coupled to COMP pin of the driver circuit PA12. The first switch PA118 is coupled to OUT pin of the driver circuit PA12.
As the first switch PA118 is conducted, an average circuit Ia is generated flowing through the LED PA116 and an inductor current Ib is generated flowing through the inductor PA117. In general, operation mode of the load circuit PA11 is decided by on time of the first switch PA118, which controls the increasing and decreasing of the inductor current Ib. The average of the peak value and the valley value of the inductor current Ib is the average current Ia. The above mentioned operation modes mainly include continuous conduction mode (CCM), discontinuous conduction mode (DCM), and boundary conduction mode (BCM), which are decided by the charging/discharging action of the inductor current Ib.
However, each of the operation modes has both advantage and disadvantage. Take the CCM mode for example, CCM mode has the advantage of small input and output ripple, small total harmonic distortion (THD) and Electra Magnetic Interference (EMI), and easier to executing filtering task, however, due to the restriction of circuit design of the load circuit PA11 in present, it is necessary to set the switching frequency or the switching time of the load switch under CCM mode. Once the setting is completed, as the inductance of the inductor PA117 has a significant change, a large ripple current would be generated even though the average current Ia is still stable. Thus, the LED with higher withstand current is needed such that the cost of the LED PA116 would be increased and thus the need to improve the technology in present exists.